Multiresolution subdivisions are creating curves down the middle of my model
Looks like you have internal faces down the middle (probably left over from a mirror modifier). Delete all the faces that are inside of your guy, they are on the center, right where the crease is. 

You can easily remove any interior faces. 


Tab into edit mode, make sure no part of your mesh is selected.
From the 3D view header Select > Interior Faces.
Switch to wireframe with Z and if you have any faces selected press X > Faces.